Elastically or plastically highly extensible self-adhesive tapes which can be redetached without residue or destruction by extensive stretching in the bond plane are known for example from U.S. Pat. No. 4,024,312 A, DE 33 31 016 C2, WO 92/11332 A1, WO 92/11333 A1, DE 42 22 849 C1, WO 95/06691 A1, DE 195 31 696 A1, DE 196 26 870 A1, DE 196 49 727 A1, DE 196 49 728 A1, DE 196 49 729 A1, DE 197 08 364 A1, DE 197 20 145 A1, DE 198 20 858 A1, WO 99/37729 A1, and DE 100 03 318 A1 and are referred to below inter alia as strippable self-adhesive tapes.
Strippable self-adhesive tapes of this kind are frequently used in the form of single-sidedly or double-sidedly pressure-sensitively adhesive sheet strips which preferably have a non-adhesive grip area from which the detachment operation is initiated. Particular applications of corresponding self-adhesive tapes are to be found inter alia in DE 42 33 872 C1, DE 195 11 288 C1, U.S. Pat. No. 5,507,464 A, U.S. Pat. No. 5,672,402 A, and WO 94/21157 A1. Specific embodiments are also described in DE 44 28 587 C1, DE 44 31 914 C1, WO 97/07172 A1, DE 196 27 400 A1, WO 98/03601 A1, and DE 196 49 636 A1, DE 197 20 526 A1, DE 197 23 177 A1, DE 197 23 198 A1, DE 197 26 375 A1, DE 197 56 084 C1, DE 197 56 816 A1, DE 198 42 864 A1, DE 198 42 865 A1, WO 99/31193 A1, WO 99/37729 A1, WO 99/63018 A1, WO 00/12644 A1, and DE 199 38 693 A1.
Preferred fields of use of aforementioned strippable adhesive sheet strips include in particular the residuelessly and nondestructively redetachable fixing of light to moderately heavy articles in the home, workplace, and office segments. In these applications they replace conventional fastening means, such as drawing pins, roundhead needles, thumb tacks, nails, screws, conventional self-adhesive tapes, and liquid adhesives, for example. Key to the successful use of the abovementioned adhesive sheet strips is not only the possibility for residueless and nondestructive redetachment of bonded articles but also the quick and easy bonding thereof and the secure hold they provide for the envisaged period of bonding. It should be borne in mind here in particular that the adhesive strips must function on a large number of substrates in order to be able to serve as a universal fixing means in the home, workplace, and office segments.
Despite the fact that the patent literature cited above describes a broad range of pressure sensitive adhesives for use in strippable self-adhesive tapes, commercial products currently on the market (for example, tesa® Powerstrips® from tesa AG, 3M Command® Adhesive strips from 3M, and Plastofix® Formuli Force 1000 adhesive strips from Plasto S.A.) all have pressure sensitive adhesives based on styrene block copolymers. Typically, use is made of linear or radial block copolymers based on polystyrene blocks and polybutadiene blocks and/or polyisoprene blocks; i.e., for example, radial styrene-butadiene (SB)n and/or linear styrene-butadiene-styrene (SBS) and/or linear styrene-isoprene-styrene (SIS) block copolymers. Advantages of the aforementioned styrene block copolymer-based PSAs for use in strippable self-adhesive tapes are, for example, the very high bond strengths which can be achieved with them (owing, inter alia, to the simultaneous realization of very high cohesion and very high adhesive forces), pronounced reduction of tack during stretching detachment (which greatly facilitates, if not indeed being a precondition for, the detachment operation), and a very high tensile strength, which is essential in particular for a detachment operation with no tearing.
A disadvantage of the majority of the products available on the market with PSAs based on the abovementioned styrene block copolymers is their low aging stability, in particular their low UV stability. This is essentially due to the use of styrene block copolymers containing polydiene (polybutadiene, polyisoprene) blocks which are unsaturated in the elastomer block. Of course, it is possible to add aging inhibitors to the PSAs used, in the form, for example, of primary antioxidants, secondary antioxidants, C-radical scavengers, light stabilizers such as UV absorbers, sterically hindered amines, or additives which scatter or reflect the damaging electromagnetic radiation, in the form for example of fillers and/or color pigments, but such additions generally produce only a gradual improvement in aging stability, so that aging processes are only delayed for a limited time.
In comparison to polymers based on unsaturated hydrocarbons, however, the aging stability achieved, in particular the UV stability realized, remains low. For example, when abovementioned, commercially available self-adhesive strips are fixed to window glass, their bond to the glass surface after just a few weeks' sunlight exposure is so strong that it is no longer possible to redetach them without residue by extensive stretching.
Accordingly it is recommended not to bond tesa® Powerstrips® to windowpanes, since the redetachability of the Powerstrips® is impaired by insolation. The Japanese in-pack information for “Command® Adhesive” strips reveals that windowpanes exposed to sunlight are an unsuitable substrate.
One way of increasing the aging stability and in particular the UV stability when using styrene block copolymers is to use styrene block copolymers hydrogenated in the elastomer block, as actualized, for example, in styrene-ethylene/butylene-styrene (SEBS; obtained by hydrogenating SBS) and/or styrene-ethylene/propylene-styrene (SEPS; obtained by hydrogenating SIS) block copolymers.
However, a disadvantage of using such styrene block copolymers, hydrogenated in the elastomer block, is that the bond strengths realizable with these copolymers are, for experience, well below those achieved with the analogous styrene-butadiene-styrene and styrene-isoprene-styrene block copolymers which have not been hydrogenated in the elastomer block. This is especially so when in the adhesive formulations employed it is necessary to minimize or forego the addition of plasticizers such as liquid resins, plasticizer oils, low molecular mass liquid polyolefins, and the like. This may be the case, for instance, when the aforementioned adhesive constituents possess a tendency to migrate from the PSA into the bonded substrates and irreversibly alter them, a characteristic which is unacceptable particularly for adhesive strips designed for the residueless and nondestructive redetachment of the bonds.
A further disadvantage of the use of PSAs based on styrene block copolymers unsaturated in the elastomer block is the fact that, for the purpose of improving their UV stability and/or of masking tinges of color which may be caused, for example, by the tackifier resins used, these adhesives are frequently pigmented. Typical pigments which enhance the UV stability and at the same time give the PSA a neutral white or specifically colored appearance include, for example, various titanium dioxides or organic and inorganic color pigments.
A drawback of such pigmentations is that the PSAs provided with them generally do not possess optical transparency or colorlessness. A consequence of the pigmentation of the PSA, for example, is a clearly visible grip-tab region, which, since it protrudes from the bond joint, is frequently found visually disruptive by the user, or else a PSA strip which owing to the pigmentation is visible over its entire area, and which is found visually disruptive, for example, in connection with the fixing of transparent substrates.
A further disadvantage of nontransparent adhesive strips is the absent possibility, when bonding one or two transparent bond partners, of being easily able to visually detect the macroscopically achieved bond area through both adhesive layers; particularly in the case of critical bonds, this is an important indicator of the quality of bonding achieved.
Numerous avenues have been pursued in order to achieve invisibility in the nonadhesive grip-tab region which is required for detachment by extensive stretching. Thus WO 98/03601 A1, for instance, describes a strippable adhesive strip which at one end contains a polymeric film—a polyester film, for example—integrated approximately in the middle of the adhesive layer. In the region of the integrated polymeric film the surface of the adhesive is made nonadhesive on both sides, by coating further polymeric films onto it or by laminating, for instance. U.S. Pat. No. 5,925,459 A describes the use of a self-adhesive strip which is redetachable by extensive stretching and which uses as its intermediate carrier a transparent elastic polymeric film. As a result of its elastic character the intermediate carrier film is able to conform to the deformation of the PSA during the detachment operation. Regions of the intermediate carrier film which are not coated with PSA serve as nonadhesive transparent grip-tab regions from which the detachment operation can be initiated and completed.
Many of the abovementioned documents incorporate the use of PSAs which in general have a high transparency and at the same time a high aging stability, in particular a very high UV stability. For example, WO 92/11332 A1, WO 92/11333 A1, and DE 195 31 696 A1 disclose the use of PSAs based on acrylate copolymers. Of these PSAs it is known, however, that the bond strengths that can generally be achieved using them, and in particular the achievable ultimate tensile stress strengths, are well below the strengths realizable with styrene block copolymer-based PSAs. Consequently, such PSAs are completely unsuitable in particular for use in single-layer strippable self-adhesive tapes, since in that case very high ultimate tensile stress strengths are vital for a reliable detachment operation.
DE 42 22 849C1, DE 197 08366 A1, DE 196 49 727 A1, DE 196 49 728 A1 and DE 196 49 729 A1 describe the utilization of styrene block copolymers based on SEBS and/or SEPS, including the use of styrene block copolymers chemically saturated in the elastomer block. However, experience has shown that, with conventional styrene block copolymers based on SEBS and/or SEPS, it is impossible to produce PSAs possessing simultaneously a high tensile strength, excellent peel strengths, high tip shear strengths, and low detachment forces (stripping forces).
A transparent UV-stable adhesive with enhanced bond strength that is suitable for use in strippable adhesive sheet strips is described in DE 100 03 318 A1.
Through the use of a special styrene block copolymer based on a hydrogenated middle block which contains nonhydrogenated polymeric side chains it is possible to produce adhesives for strippable adhesive sheet strips.
For the bonding of heavy articles such as hooks for hand towels or textiles, however, adhesives described above are not suitable, since, although the adhesive performance is increased distinctly as compared with that of adhesives comprising SEBS or SEPS, it still lies a considerable way below the bond performance of adhesives based on SIS or SBS.
It is an aim of the invention, therefore, to provide an adhesive which is suitable for a pressure sensitive adhesive strip which both is UV-stable and has a higher bond performance than existing systems, especially transparent UV-stable strippable systems, and which as far as possible is itself transparent.